Light management in photovoltaic cells is a critical area of research in order to minimize material utilization while achieving high efficiencies. By using 3-D nanostructured templates such as nanowires (NWs), the absorption length required to fully utilize the solar spectrum can be effectively decoupled from the carrier extraction length, which should be minimized to reduce recombination. Furthermore, light-scattering and trapping has been observed in 3-D architectures such as microwire, nanowire, and nanocone arrays, allowing for enhanced absorption and suppressed reflection. This permits the use of significantly less material than a planar architecture, and reduces the purity and morphological requirements of the absorber layer due to a decreased carrier extraction length. This is the principle employed in dye-sensitized solar cells (DSCs) and extremely-thin absorber (ETA) cells, which utilize a very thin absorber layer in conjunction with a nanostructured template.
Ultimately, the large-scale incorporation of photovoltaics as a renewable energy source will depend on the ability to drive down costs. While nanostructured architectures can provide several benefits for solar cells, the use of top-down processing techniques involving lithography and/or etching, or catalyst-mediated growth techniques, can add significantly to the manufacturing costs. Catalyst-based techniques can also lead to incorporation of impurities into the NWs, which can negatively affect their recombination properties.
There are a variety of growth techniques for nickel silicide NWs, most of which can be divided into three categories: delivery of silicon to nickel film, delivery of nickel to silicon NWs, and simultaneous delivery of silicon and nickel. Some new methods, such as point contact reaction between Si and Ni NWs were recently reported. Due to its simplicity and low cost, chemical vapor deposition of silane on nickel films, as the earliest discovered method, has been intensively studied.
The underlying mechanism for nanowire growth based on thermal decomposition of silane on Ni films is still a matter of debate. Nickel diffusion into silicon has been proposed by several groups, and the formation of NWs rather than films has been shown to be a strong function of temperature, pressure, and silane concentration. It has been proposed that a low supersaturation degree of the vapor-phase precursors can lead to the spontaneous formation of NWs rather than thin films, due to limited nucleation kinetics. The role of surface oxides on the growth has also been investigated, and it has been shown that the presence of surface oxide species can promote nanowire growth. The surface oxide layer can serve as either a Ni diffusion barrier, or as a catalyst for a vapor-liquid-solid (VLS) growth mechanism. However, the exact role of surface oxides on nanowire growth is not clear, and several reports have been presented without the purposeful introduction of oxygen.
What is needed is a low-cost and reduced complexity method of fabricating single-crystalline metal silicied nanowieres for anti-reflective electrodes for photovoltaic applications.